This invention relates to high-intensity-discharge (HID) lamps and, more particularly, to HID lamps of the mercury-metal halide type which efficiently illuminate objects with an excellent color appearance.
Mercury-metal halide HID lamps are described in U.S. Pat. No. 3,234,421 dated Feb. 8, 1966 to Reiling. Such lamps incorporate selected metal halide as a part of the discharge-sustaining constituents and these halides can be varied to vary the emission spectrum of the lamp. The mercury is present in a predetermined amount as required to provide a predetermined voltage drop between the lamp electrodes when the lamp is normally operating and the mercury also contributes some selected visible emissions to the composite light output of the lamp. Possible metal halides which can be used are specified in this patent as iodides of lithium, sodium, cesium, calcium, cadmium, barium, mercury, gallium, indium, thallium, germanium, tin thorium selenium, tellurium and zinc.
In U.S. Pat. No. 3,407,327 dated Oct. 22, 1968 to Koury et al., is disclosed a mercury-metal halide HID lamp which incorporates sodium iodide and scandium iodide as the principal discharge-sustaining and light-emitting constituents. Commercial embodiments of such lamps are used extensively in stadium lighting and area lighting as well as some interior-type applications. The combined sodium and scandium discharge together with the mercury generates what can be described as forest of lines of visible emission which attempts to duplicate the effect of a continuous spectrum. Such lamps are noticeably deficient in the red region of the spectrum, however, which impairs the color appearance of objects which are illuminated by these lamps.
In U.S. Pat. No. 3,521,110 dated July 21, 1970 to Johnson is disclosed a mercury-metal halide type lamp wherein one or more of a large group of specified halides have added thereto selected halides of tin, lead, antimony or bismuth in order to provide what is termed a regenerative halide cycle, with stannous halide being preferred. U.S. Pat. No. 3,781,586 dated Dec. 25, 1973 to Johnson discloses adding elemental tin to the discharge-sustaining filling in order to prevent the existence of free iodine. A discharge-sustaining filling including both stannous chloride and stannous iodide together with sodium halide is disclosed in U.S. Pat. No. 3,882,345 dated May 6, 1975 to Kazek et al. Such a lamp displays a continuous spectrum of the tin discharge with broadened sodium line radiations superimposed thereon. Lighting Research and Technology, Volume VIII, No. 3 (1976), pages 136-140, article by Lorenz, discloses that red emission can be obtained when calcium iodide is included in a mercury-metal halide lamp. To enhance the emission properties of the calcium iodide, aluminum chloride is included therewith.
In Journal Light & Vis. Env., Volume 1, No. 2 (1977), pages 5-19, article by Ishigami et al., is disclosed a mercury-metal halide lamp wherein the red emission of a tin-halide continuous spectrum is augmented by the molecular emission of calcium halide. The predominate material which contributes to the light emission from the discharge remains the tin halide, however, and the additive calcium halide enhances the color rendering properties of the lamp by adding some needed red emissions. A U.S. Pat. No. which corresponds to this publication is 4,027,190, dated May 31, 1977, wherein the continuous tin spectrum is augmented in the red regions of the spectrum by a small calcium halide emission superimposed thereon. In the foregoing art, the tin-halide discharge is shown as predominating the lamp emission characteristics, with the calcium halide only used as an additive to improve the coloring rendering properties of the composite lamp emission.
Summarizing the development of the mercury-metal halide lamps to date, it is probably safe to say that every metal which will form a vaporizable halide has been tried as a discharge-sustaining constituent, along with numerous combinations of metal halides. The majority of these efforts have been directed toward producing a composite discharge which resembles a continuous spectrum, in order to simulate the illumination of natural light as closely as possible.
In U.S. Pat. No. 4,176,299, dated Nov. 27, 1979 to W. A. Thornton, one of the present applicants, is disclosed a light source which generates three narrow bands or lines of selected radiations, namely, blue-violet radiations peaked at about 450 mm, green radiations peaked at about 540 nm and a red-orange radiations peaked at about 610 nm. When these individual bands or lines of radiations are blended, the color appearance of illuminated objects is excellent. In U.S. Pat. No. 3,875,453 dated Apr. 1, 1975 is disclosed a fluorescent lamp having high color-discrimination capability. This is achieved by concentrating the emissions in the wavelength ranges of 400-470 nm, 500-550 nm and 610-680 nm with the relative proportions of the emissions selected to produce visible light of predetermined ICI coordinates.
In recent years, a color-preference index has been proposed for rating the performance of light sources in accordance with what the normal observer considers to be the preferred coloration for familiar objects. This color preference index (CPI) is summarized in the Journal of the Illuminating Engineering Society, pages 48-52, October 1974, article entitled "A Validation of the Color-Preference Index" by W. A. Thornton, one of the present applicants.